1. Field of the Invention
The present invention relates to radio-frequency (RF) or intermediate-frequency (IF) devices and, in particular, to a method and an apparatus for compensating for non-linear behavior of RF/IF devices.
2. Description of the Related Art
The surge in popularity of mobile phones has prompted the development of a wide variety of RF/IF devices for use in wireless communication systems. Testing and characterization of RF/IF devices play a crucial part in their development, as system designers must accurately assess the performance of these devices in a wireless network. Such testing often involves applying an unmodulated RF carrier to the device and analyzing the response signal generated by the device. The operating characteristics of the RF/IF device are assessed in accordance with the phase and amplitude of the response signal as compared to the phase and amplitude of the input carrier.
RF/IF devices exhibit non-linear behavior, which can be detrimental to the performance of wireless communication equipment. For example, the output signal frequency of these devices may undergo frequency shift (e.g., with mixers) or include frequencies in addition to those of the input signal. There is, however, no prior art method and apparatus for linearizing output signals of RF/IF devices.
An object of the present invention is to provide a method and apparatus for compensating for non-linear behavior of RF/IF devices for wireless communication systems.
According to one aspect, an adaptive compensating device, in combination with an RF/IF device, includes a digital filter for compensating for non-linearity of the RF/IF device by preprocessing an input signal prior to transmission to the RF/IF device so that an output signal from the RF/IF device is substantially linear with respect to at least one of phase and amplitude of the input signal.
According to another aspect of the invention, for compensation of amplitude ripple, the digital filter is a finite impulse response (FIR) filter. For compensation of group delay, the digital filter is an infinite impulse response (IIR) filter. For compensation of error vector magnitude, the digital filter is a complex filter.
In one embodiment, an apparatus for computing coefficient values for configuring a digital filter for compensating for non-linearity of an RF/IF device includes a reference signal generator, connected to an input of the RF/IF device, for generating an input reference signal to the RF/IF device. A vector signal analyzer, connected to an output of the one of a radio frequency and intermediate frequency device, analyzes a response signal from the RF/IF device and outputs digital in-phase and quadrature-phase (I/Q) signals. A processor, connected to an output of said vector signal analyzer, computes a difference between the input reference signal and the digital in-phase and quadrature-phase (I/Q) signals. The processor calculates the coefficient values for configuring the digital filter based on the computed difference. A memory device, connected to the digital filter, stores the calculated coefficient values for the digital filter.